30C/20min = 30C/1200sec
= 0.025 C/sec
= 0.025 Ampere of current.
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I can't imagine where you got this question, and I'd very much like to know.
It says it's describing a lightning bolt, but it's got the current about a million times too small, and the time at least a million times too long. Also, the voltage doesn't matter, but it's also in the neighborhood of a million times too small for lightning. It's actually more like 108 volts FOR EVERY FOOT OF DISTANCE between the cloud and the ground.